Gaseous fuel internal combustion engines have long been known, and are increasingly commonplace in today's society. A typical gaseous fuel internal combustion engine differs from a traditional, liquid fuel internal combustion engine primarily in that a gas, such as methane, natural gas, ethane, propane, etc., or some mixture, is burned in the engine rather than an atomized mist of liquid fuel from a fuel injector or carburetor. Most gaseous fuel engines are spark ignited by a conventional J-gap spark plug. Other variations such as a non-enriched pre-chambered spark plug or an enriched pre-chamber with J-gap spark plug can also be used to ignite the gaseous fuel and air mixture. In other examples, such as dual fuel engines, the gaseous fuel is ignited via compression ignition of a small pilot injection of distillate diesel fuel that propagates a flame front and burns the gaseous fuel and air mixture. While it may be conceivable to produce a compression ignition gaseous fuel engine, no commercially viable compression ignition gaseous fuel engines are known to exist. The use of a gaseous fuel rather than a liquid such as gasoline or diesel presents challenges in regulating the amount of fuel supplied to the engine. For example, it is relatively easier to inject a discrete quantity of liquid fuel directly into an engine cylinder or combustion pre-chamber than to deliver a measured charge of combustible gas, in certain engines. One of the reasons for this fuel metering challenge in gaseous fuel engines relates to the volume and/or pressure changes undergone by gases with changes in temperature.
Nevertheless, gaseous fuel engines can offer significant advantages, one of which is a reduction in certain exhaust gas pollutants. For instance, an internal combustion engine that burns a gas such as methane emits very little, if any unburned hydrocarbon materials or soot. Gaseous fuel internal combustion engines may also be better suited than traditional liquid fuel engines to remote environments where a supply of combustible gas such as natural gas is available, but refined hydrocarbon fuels are cost ineffective or unavailable altogether.
Some pollutants inevitably result from the burning of hydrocarbons as fuel, whether gaseous or liquid. Engineers have devised many ways to reduce certain pollutants in engine emissions over the years. Sophisticated control over fuel injection quantity and timing, fuel additives and catalytic converters all represent attempts to improve the economy and emissions profile of various internal combustion engines.
While substituting gaseous hydrocarbons for liquid hydrocarbons in an internal combustion engine offers inherent advantages, engineers are continually seeking improvements. One class of pollutants of concern is known generically as NOx. NOx refers to several types of nitrogen-oxygen compounds, varying in the number of oxygen atoms bonding with a single nitrogen atom in each molecule.
One attempt to reduce emission of NOx compounds in an internal combustion gasoline engine is known from U.S. Pat. No. 4,173,205 to Toelle. Toelle describes a system wherein a closed loop exhaust gas recirculation system pumps exhaust gas from the engine into the engine intake manifold. The Toelle system is electronically controlled, and utilizes a look-up table having supposed optimal values for manifold air pressure for a given throttle position and engine speed. An electronically controlled valve in the exhaust gas recirculation system is adjusted to provide relatively more or less exhaust gas recirculation flow quantity as needed to reduce NOx emissions. Toelle teaches one attempt to reduce NOx in an internal combustion engine, however, the design is not without its shortcomings, primarily in that manifold air pressure alone represents only an approximate predictor of NOx content in the engine exhaust.
Another known design for limiting NOx production is taught in U.S. Patent Application Publication No. 2004/0024518 to Boley et al. Boley et al. teach a system wherein a density of a combustion mixture entering an engine is adjusted to adjust a NOx output thereof. Boley et al. teach the use of a mass flow sensor or the combination of a pressure and temperature sensor to determine a density of the combustion mixture. Once known, the combustion mixture density can be adjusted to a desired level by increasing fuel flow and/or air flow into the engine. While the Boley et al. design offers certain advantages, the density of the combustion mixture is adjusted only by adjusting the relative proportions of air to fuel in the mix, which may limit the engine to certain operating schemes.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.